McConnell, et al. have developed an S parameter method of spectral interpretation almost universally used in the analysis of spin-labeled membrane spectra. Hubbell and McConnell have emphasized that for their S formalism to be valid, one must have Trace T equals Trace T', where T is the nitrogen hyperfine tensor in the principal axis system and T' has the components T' and T'. This requirement follows directly from the rotational invariance of the trace of a tensor. In the work of Hubbell, et al. variations in aN, the isotropic nitrogen hyperfine splitting, have been interpreted as changes in the polarity of the local environment of the nitroxide group, and a correction has been used to adjust the measured S for the polarhydrophobic effect. This polarity profile within the hydrocarbon region of the bilayer has been attributed to electrostatic interactions between the nitroxide dipole and the dipolar regions of the bilayer. Although the polarity profile does probably exist in many cases, a breakdown of assumptions made in the derivation of the S parameter formalism could cause an artifactual polarity change which is actually a violation of the necessary condition that Trace T equals Trace T'.